In-situ generation of miscible gas bank



Nov. 26, 1963 J. ORKISZEWSKI 3,111,937

INSITU GENERATION OF MISCIBLE GAS BANK Filed Nov. 15, 196].

JOSEPH ORKISZEWSKI INVENTOR.

AGENT United States Patent 3 111 987 iN-srru GENsnArrois oF MISCIBLE GAS BANK Joseph (lrkiszewski, Tulsa, Okla, assiguor to Jersey Production Research Company, a corporation of Delaware Filed Nov. 15, 1961, Ser. No. 152,489 4 Claims. (Cl. 166--11) The present invention is concerned with the recovery of petroleum from subterranean reservoirs. Broadly, the invention is directed to a method for increasing the chiciency of petroleum recovery from reservoirs amenable to gas pressure maintenance operation, by establishing at the gas-oil interface a fluid bank that is miscible both with the reservoir oil and the overlying body of gas. More particularly, the invention involves a method for the insitu generation of the miscible fluid bank by underground combustion.

The invention is applicable in reservoirs having a gasoil interface and wherein the oil is susceptible to gravity drainage. One or more wells are provided in communication with the oil zone near the gas-oil interface. To generate the miscible fluid bank, in-situ combustion operations are conducted at these wells, injecting oxygen at flux rates sufhciently high to sustain combustion. All such Wells are operated concurrently as oxygen injection wells, thereby necessitating the generated combustion gases to migrate upward to the gas cap. Oxygen injection is continued until the combustion front has progressed as far as practical from the injection wells.

The burning is continued over a period of several weeks, up to a year or more, depending on the size of the reservoir and the rate at which the burning progresses. At this time the wells are shut in and condition within the reser' voir allowed to equilibrate. As the oil flows back into the burned out zone it is heated to a temperature at which some portion of the oil is cracked. The cracking results both in the generation of miscible gases, as well as coke which can further heat the formation on subsequent burning cycles. That is, when the temperature falls below the threshold of cracking, the wells are preferably immediately re-ignited and the burning step repeated.

Since the second burning cycle is initiated at a time when the reservoir is at a temperature of 500 to 600 F. the combustion will raise the temperature of the reservoir to a level considerably higher than that obtained upon the first burning cycle. At the termination of the second burnin cycle, temperatures within the reservoir of 1,500 F. or more are readily attained. Hydrocarbons backflowing into the burned-out area after the second burning cycle are heated to temperatures considerably in excess of the minimum temperature necessary for cracking. The result is hi h-severity cracking which produces large amounts of miscible gases which migrate to the gas-oil interface thereby forming a miscible fluid layer.

Carbon dioxide produced in the combustion process forms a substantial fraction of the miscible gas bank. Therefore, commercial-grade bulk oxygen (about 90% 0 by volume) is preferred over air or oxygen-enriched air as the injection gas, since it is desirable to avoid any unnecessary dilution of the C0 generated by the combustion.

t is known in the prior art that the injection of a miscible fluid capable of forming a layer at the gas-oil interface increases the efficiency of petroleum recovery from the reservoir by recovery methods which are otherwise conventional. By way of explanation, it has been postulated that the increased efficiency arises from migration of antes? Patented Nov. 26, 1963 tact is found in US. Patent 2,885,003 to R. L. Lindauer, Jr., which was reissued as Reissue Patent 24,873. The present invention provides a novel method for forming such a miscible fluid bank within the reservoir at the gasoil interface.

Underground combustion within a petroleum reservoir, as such, is well known. It has been used as a method for direct thermal displacement of relatively immobile oil, retained in a reservoir after primary depletion. In contrast to such prior methods, the present invention utilizes underground combustion during primary depletion, and depends upon gravity drainage of the reservoir oil for successful operation.

In-situ combustion, as it is conventionally practiced, utilizes an existing well communicating with the oil reservoir as an input well and employs one or more existing wells spaced therefrom as the producing well. It involves the initiation of combustion along the bore of the input well. After ignition, air or oxygen is continuously injected into the formation with the result that a burning wave or front gradually advances toward the producing well or wells. The principal fuel which supports the combustion is a carbonized deposit formed in-situ immediately ahead of the burning front. Upon continued combustion heated gaseous products are developed, which under the existing pressure are forced into the oil reservoir ahead of the burning front. The heat imparted in this manner to the oil in the sand volatilizes a portion of oil and reduces the viscosity of the remaining heavier constituents of the oil. The vaporized portions of the oil are entrained by the stream of gaseous products of combustion and move therewith toward the producing well while the fluidized heavier portions of the oil are mechanically forced under pressure of the gaseous products of combustion toward the producing well or wells. As the vaporized portions of the oil move into cooler regions of the oil containing sand they are partially condensed and release the latent heat of condensation at that point which together with the sensible heat in the gaseous products of combusion serves to increase the temperature in regions of the sands remote from the input well.

Thus the entire oil reservoir is progressively heated and the oil in vaporous and/ or liquid state is forced into the producing well bore where it is removed by ordinary pumping means. Vaporization of a portion of the oil and in addition formation of steam from the connate water adds to the total volume of gases facilitating removal of the oil from the reservoir. A more detailed review of the in-situ combustion process is found in the July 8 issue of The Petroleum Engineer, beginning on page B29.

A detailed description of the invention is provided by reference to the following drawing. A dipping reservoir 11 is penetrated by wells 12, 13 and 14. The reservoir is characterized by gas cap 15 and original gas-oil interface 15. The reservoir may be virgin or it may be par tially depleted; but at any rate, permeability characteristics of the reservoir and the viscosity of the oil must be such that gravity drainage will result.

In accordance with the invention, one or more wells 14 near the gas-oil interface are utilized as oxygen injection wells for the purpose of initiating combustion within the reservoir. Combustion is initiated by conventional means such as down-hole electrical heaters, gas burners or chemical oxidation methods. Frequently, ignition occurs spontaneously upon the injection of oxygen. Oxygen flux rates are maintained in excess of 0.25 standard cubic foot per hour per square foot of combustion front in order to ensure the maintenance of a combustion front within the reservoir. The temperatures created within the reservoir upon initial burning range from as little as 600 to as high as l200 F. at the actual burning front. Temperatures behind the front gradually decline.

Once the combustion front has advanced a suflicient distance from the injection well so as to cause the flux rates at the burning front to fall below the rate necessary to sustain combustion, the well or wells 14 are shut in and the pressure within the reservoir is allowed to equilibrate. The combustion gases are forced to migrate to the gas cap since no wells in the combustion vicinity are produced. While the wells are shut in, oil backfiows by gravity into the burned area 17 where the temperature is sufiicient to cause a cracking reaction. The gases produced by cracking include substantial quanties of light hydrocarbons, predominantly olefins. These gases migrate upward by gravity forces to the gas cap. These gases diffuse into the gas cap, spreading across the gas-oil interface, where an equilibrium is established, resulting in the formation of a layer of light oil 18 at the interface comprising reservoir crude containing dissolved cracked gases. The shut-in period of back-flow and cracking is continued for a few weeks, up to several months or more, depending upon the extent of burnout and the mobility of the crude. 'Eroduction of the reservoir from wells at positions 12 and 13 causes fluid layer 18 to slowly traverse the reservoir, moving down dip as fully described in the above mentioned Lindauer patent.

Operation in this manner, however, is not entirely satisfactory since the temperatures achieved in such a burn out are not high enough to cause sufficient cracking upon back-flow to create a miscible layer 18 which is thick enough and light enough to fulfill its intended purpose. Therefore it is usually necessary at the end of backlow subsequent to the first burning cycle to immediately re-ignite well or wells 14 in order to carry out a second burning cycle in the same region of the reservoir as is initially burned out. second burn-out is the coke and other deposits laid down during the initial cracking period. Temperatures within the reservoir at the beginning of such a second combustion cycle are in the vicinity of 600 F. Accordingly, the second burning cycle raises the temperature to several hundred degrees higher than was possible during the first burn-out. At the end of the second burning cycle when hydrocarbons back-flow into the burned out area much more severe cracking occurs, leading to the formation of greater amounts of cracked gases which in turn migrate to form a miscible bank 18 which has the desired thickness and a substantially reduced viscosity. Once such a miscible layer is created, production down dip is continued until depletion of the reservoir, without the need for further burning cycles, as more fully described in the above mentioned Lindauer patent.

In some reservoirs of course, it may be necessary to conduct still a third or subsequent burning cycle in the same area of the first two cycles. The need for more than one burning cycle arises because the temperatures achieved during initial burn-out are seldom wholly adequate to create the desired quantity of cracked products.

While the process of the invention is being carried out, it is not necessary to interrupt the production schedule of down-structure wells 12 and 13. That is, the density difference which forces injected gas and combustion gases up-structure from well 14 is more than sufficient to ensure that production down-structure is unaffected by the combustion process.

As an example of the invention, assume that the process is carried out in a dipping gas cap reservoir, that has an area of 1200 acres and a net oil sand thickness of 30 feet wherein the oil zone has a porosity of 25%, a permeability 800 millidarcies, and is 75 percent saturated with crude having a viscosity of centipoises. The reservoir pressure is 2500 p.s.i., and the temperature is 150 F.

Three existing wells, completed at distances of at least 1200 feet from gas-oil interface, are employed concurrently as oxygen injection wells. Burning is initiated sur- Principal fuel for the rounding each wellbore, and bulk oxygen analyzing volume per cent 0 is injected at a rate of 44 MM s.c.f. per day. Oxygen injection is continued for 7 months at which time the wells are shut in to allow the oil to drain back into the burned-out region of the reservoir. After 3 months have lapsed, the wells are opened and re-ignited. Oxygen is again injected at 44 MM s.c.f. per day for 7 months, whereupon the wells are again shut in to equilibrate. Back-flowing hydrocarbons are subjected to severe cracking. Calculations based on the heat content of the burned-out region and the thermal requirements of the cracking process show that a miscible bank is formed with a volume of 14.89 l0 bbls. which fills 21.2% pore volume of the reservoir. The miscible gas bank has the following volumetric composition:

N 6 1 Methane 20.8 Carbon dioxide 54.4 Ethane and ethenes 8.6 Propane and propenes 6.7

Butane and butenes 3.4

Carbon dioxide exhibits phase behavior quite similar to ethane and ethenes. The wet gas bank would contain 73.1% intermediates and a dry gas make-up (N and methane) of 26.9%. Benhams correlations 1 show that for the reservoir conditions of F. and 2500 p.s.i. with a pseudo molecular weight of 31 (equating carbon dioxide as ethane) for the intermediates, the bank can contain dry gas volumes up to 48% and still maintain miscibility.

The miscible bank having thus been established, it slowly migrates down-dip under natural reservoir gas drive, which may be augmented by dry gas injection in the crystal structure wells.

What is claimed is:

1. In the recovery of petroleum from a substerranean' earth reservoir characterized by a gas cap and a gasoil interface, and wherein a miscible fluid layer is estab lished at said gas-oil interface for the purpose of increasing ultimate recovery from said reservoir, the improved method of forming said miscible fluid layer which comprises initiating combustion within said reservoir adjacent an injection Well therein near said gas-oil interface, injecting oxygen through said injection well into the region of combustion in order to sustain said combustion for a substantial period, whereby the temperature of said reservoir is raised above the threshold of cracking, shutting in said injection well for a period of time suiticient to permit back-flow of reservoir hydrocarbons by gravity drainage whereby thermal decomposition of said hydrocarbons results, with a consequent migration of cracked products to said gas cap whereby said miscible layer is formed, and producing petroleum from said reservoir through a production well therein.

2. A method as defined by claim 1 wherein combustion is re-initiated subsequent to said cracking, followed by again shutting in said injection well thereby causing a second cracking cycle of greater severity than said first cracking, and then producing hydrocarbons from said reservoir.

3. A method for recovering petroleum from an underground reservoir, said reservoir being amenable to gas pressure maintenance and gravity drainage, which comprises injecting oxygen into said reservoir through an injection well therein at a point near the upper extremity of the oil bearing zone under conditions whereby in-situ combustion is initiated and maintained, interrupting said oxygen injection after a substantial volume of said reseryou is burned out, allowing the displaced oil to flow back into said burned out volume by gravity drainage, whereby a portion of said oil is cracked to produce light hydro- Benham, if L., Miscible Fluid Displacement-Prediction of Misc'ibihty -Journal of Petroleum Technology Octohcr 1960, page 220.

carbon gases which migrate to the top of the oil bearing zone thus forming an oil-miscible layer of reduced density and viscosity and thereafter withdrawing oil from said reservoir through a production Well therein whereby said miscible layer is caused to migrate through said reservoir enabling an increased efficiency of petroleum recovery to be realized.

4. A method for recovering petroleum from an underground reservoir, said reservoir being amenable to gas pressure maintenance and gravity drainage which comprises (l) injecting oxygen into said reservoir through an injection well therein at a point near the upward extremity of the oil-bearing zone under conditions whereby in-situ combustion is initiated and maintained, (2) interrupting said oxygen injection after a substantial volume of said reservoir is burned out, (3) allowing the displaced oil to flow back into said burned out volume by gravity drainage whereby a portion of said oil is cracked to produce coke and light hydrocarbon gases which migrate to the top of the oil bearing zone thus forming an oil- References Cited in the file of this patent UNITED STATES PATENTS Re. 24,873 Lindauer Sept. 27, 1960 2,584,606 Merriam et al Feb. 5, 1952 2,788,071 Pelzer Apr. 9, 1957 3,036,632 Koch May 29, 1962 OTHER REFERENCES McNiel, J. 8., Jr., and Nelson, T. W.: Thermal Methods Provide Three Ways to Improve Oil Recovery, The Oil and Gas Journal, vol. 57, No. 3 (January 19,

1959) pages 86-98 (page 94 relied on). 

1. IN THE RECOVERY OF PETROLEUM FROM A SUBSTERRANEAN EARTH RESERVOIR CHARACTERIZED BY A GAS CAP AND A GASOIL INTERFACE, AND WHEREIN A MISCIBLE FLUID LAYER IS ESTABLISHED AT SAID GAS-OIL INTERFACE FOR THE PURPOSE OF INCREASING ULTIMATE RECOVERY FROM SAID RESERVOIR, THE IMPROVED METHOD OF FORMING SAID MISCIBLE FLUID LAYER WHICH PROVED METHOD OF FORMING SAID MISCIBLE FLUID LAYER WHICH COMPRISES INITIATING COMBUSITON WITHIN SAID RESERVOIR ADJACENT AN INJECTION WELL THEREIN NEAR SAID GAS-OIL INTERFACE, INJECTING OXYGEN THROUGH SAID INJECTION WELL INTO THE REGION OF COMBUSTION IN ORDER TO SUSTAIN SAID COMBUSTION FOR A SUBSTANTIAL PERIOD, WHEREBY THE TEMPERATURE OF SAID RESERVOIR IS RAISED ABOVE THE THRESHOLD OF CRACKING, SHUTTING IN SAID INJECTION WELL FOR A PERIOD OF TIME SUFFICIENT TO PERMIT BACK-FLOW OF RESERVOIR HYDROCARBONS BY GRAVITY DRAINAGE WHEREBY THERMAL DECOMPOSITION OF SAID 